The present invention relates to a hydraulic control system for a continuously variable transmission provided with a torque converter having a lockup clutch, and more particularly to a circuit for supplying oil to an oil cooler and lubricating oil to various parts of the transmission system.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by the fluid operated servo device so as to vary a running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a secondary pressure control valve and a primary pressure control valve. Each valve comprises a spool which controls the oil supplied to the servo devices.
In the hydraulic circuit, a large quantity of oil is necessary to be applied not only to the transmission but also to the torque converter, a lockup clutch is provided for the torque converter, a selector device selects the forward driving and reverse driving, and lubricates the parts such as the belt of the transmission. The hydraulic circuit is adapted to supply an adequate quantity of the oil to each part while maintaining an oil balance in the circuit. Oil temperature is increased in the torque converter. In order to reduce the oil temperature, the oil cooler is disposed in a drain passage of the torque converter. It is necessary to supply a sufficient quantity of the oil to the oil cooler to maintain sufficient cooling efficiency although the passage of the oil changes to another passage in accordance with the operation of the lockup clutch. Furthermore, the belt must be lubricated without affecting the overall oil balance.
Japanese Patent Application Laid Open 62-137462 discloses a hydraulic circuit wherein the oil cooler is provided in a bypass passage from a regulator valve. The oil drained from the regulator valve is supplied to the lockup clutch, the torque converter and the selector device. Oil pressure applied to the devices are controlled to maintain an appropriate value.
The lubricating oil for the transmission is splashed on, or applied from the drain of the oil cooler to various parts of the transmission system. However, in order to splash up the oil, the oil is agitated, which causes a load on the transmission. Consequently the transmission efficiency is decreased and the quantity of the oil used is increased. As a result, fuel consumption and performance of the engine deteriorate. In another way, when the drain oil from the cooler is used, orifices and other devices are necessary to branch lubricating passages and to control the flow of the oil. The orifices cause increased resistance of the passage so that the oil is not sufficiently cooled. Moreover, the inner pressure of the oil cooler increases when the oil temperature is low, resulting in the cooler breaking down. If all the drain oil is used as the lubricating oil, the same defects as those caused by splashing occur.